This invention relates generally to an optical coupler for launching laser light into a multi-mode optical fiber. Present methods for launching laser light into a multi-mode fiber bus include a fused multi-mode four-port star coupler and polished and cemented junctions. Another multi-mode coupler involves the modification of a PIN-TAP.TM. device (manufactured by ADC Magnetic Controls Company of Minneapolis, Minn.). A PIN-TAP.TM. device consists generally of a fiber optic bus having a partially reflecting mirror positioned at about a 45 degree angle to the core axis in a multi-mode fiber. Light traveling down the multi-mode fiber is partially reflected at the mirror to a PIN diode where it is converted into an electrical signal. A reverse PIN-TAP coupler is obtained by modifying the PIN-TAP device by replacing the PIN diode with a laser diode and a lens. The lens focuses the laser light onto the partially reflecting mirror and into the multi-mode fiber bus.
The major drawback to the star coupler is modal noise for small coupling ratios, and for the polished and cemented junction is additionally the lack of stability of the optical coupling ratio. The reverse PIN-TAP type coupler overcomes the modal noise problem with the partially reflecting mirror which injects light into many possible modes simultaneously. The desired amount of light injected into the fiber bus is determined by the percent of light reflected at the mirror. However, each of the three types of couplers has a very large loss in signal strength for either the injected signal or the signal travelling in the bus. Depending on the number of stations or couplers in a particular application, the optmum degree of reflectivity can range from 0.1% to 50%. For example, a partially reflective mirror chosen to inject only about one percent (1%) of the input laser energy transmits about 99% of the optical power in the fiber bus. Low overall coupling efficiency is required in order to keep insertion loss low for the signals in a fiber bus having many such couplers.
The present reverse PIN-TAP coupler is a design capable of injecting only about 8% of the input laser energy ideally reflected from the mirror. Therefore, it is an object of the present invention to provide a multi-mode coupler, with nearly 100% of the coupling efficiency permitted by the chosen mirror's reflectivity, for launching laser light into a multi-mode fiber.
It is another object of the present invention to provide an efficient, low-loss means to couple laser light into a multi-mode fiber bus.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.